1. Field of the Invention
The present invention relates to a method for curing carbonate monomers, e.g., diethylene glycol bis (allyl carbonate) using electromagnetic radiation.
2. Discussion of the Background
Diethylene glycol bis (allyl carbonate), (DADC), is a monomer which is widely used as an optical plastic for production lenses, safety glasses and guards, watch crystals, and instrument windows. The most common method of preparing DADC is by the reaction of diallyl carbonate and diethylene glycol in a relative molar ratio higher than about 10:1, in the presence of a basic catalyst, for example, as disclosed in U.S. Pat. No. 4,508,656 and U.S. Pat No. 4,623,705. This reaction scheme produces at least about 80-90% by weight pure DADC. If the ratio of diallyl carbonate to diethylene glycol is on the order of 2:1, an oligomer or a mixture of oligomers of diethylene glycol bis (allyl carbonate) is produced.
DADC, which has formula (I), polymerizes via a free radical mechanism using a suitable polymerization initiator to form a thermosetting polycarbonate. One of the most common initiators is benzoyl peroxide. The polymerization process involves a long initial curing cycle, typically at least 48 hours, at temperatures below 100.degree. C. The initial curing is followed by a post-curing cycle at 110.degree.-120.degree. C. for several hours to destroy the unreacted catalyst as well release any stresses produced in the polymerization step. Above 100.degree. C., benzoyl peroxide (BPO) cannot be used as an initiator because it decomposes too rapidly at these temperatures. Other initiators are available for polymerization at lower temperatures, as described in U.S. Pat. No. 4,607,087. ##STR1##
Polymerization of DADC results in a clear, colorless, abrasion-resistant polymer casting. The bulk polymerization is slow at first. With increasing conversion and upon gelation, polymerization is accelerated. The polymerization is highly exothermic. Thus, to prevent a runaway reaction with overheating and bubbling from occurring, especially in large pieces, a cooling system must be present. After the exothermic reaction, the rate of polymerization slows down.
Due to the slow reaction rate of the monomer, curing cycles on the order of 48-72 hours are used to obtain complete conversion of the monomer to polymer when using benzoyl peroxide as initiator. Even when using more reactive initiators, such as peroxydicarbonates, cycle times of 20 hours are commonly used. The long curing times which are associated with the above-mentioned conventional thermal curing methods for these materials limits their applications.
It is well known that high energy ionizing radiation, such as X-rays, electron beams from betatrons, cyclotrons, and other high energy electron sources or radiation from radioactive elements, such as cobalt 60, can cause chemical reactions. These reactions occur as a result of ionization which is induced in the reagents irradiated, and require energy on the order of several electron volts. At lower frequencies in the microwave range, the energy of the electromagnetic radiation is much too low to produce ionization and hence the chemical reactions that are obtained with ionizing radiation cannot generally be obtained with electromagnetic radiation in the radio-frequency and microwave range. Electromagnetic radiation in these frequency ranges, however, has been employed to create a heating effect in irradiated dielectric materials, e.g. dielectric heating and diathermy. The thermal effect caused by electromagnetic radiation in the radio-frequency and microwave range has also been utilized in chemical reactions which are activated by heating. In particular, the setting of adhesives or the curing of certain resin compositions may be accomplished through the dielectric heat caused by electromagnetic irradiation in this frequency range.
U.S. Pat. No. 3,432,413 discloses a method of initiating and conducting chemical reactions in a two-component system using non-ionizing radiation. Chemical reactions, in particular the polymerization of vinyl monomers, are conducted using pulsed non-ionizing electromagnetic radiation in the radio-frequency and microwave range to avoid substantial heating and undesired thermal effects.
According to U.S. Pat. No. 3,432,413, electromagnetic radiation in the radio-frequency and microwave range can initiate the polymerization of vinyl monomers without the use of chemical initiators only when a second component is present which is inert with respect to the monomer and to the polymerization, and which has a polarity different from that of the vinyl monomer. The required use of an inert diluent is expensive and has the disadvantage of requiring additional process steps to separate the diluent from the desired product.
A need continues to exist, therefore, for improved methods of producing polymers of carbonate monomers which do not have the limitations of existing methods.